I think they want to call it X-Pop is because it is a K-Pop group but with American influence in their Music.
If you listen to K-Pop and J-Pop you'll know eventually which one is which but with XG although they use the K-Pop system, the way they produced Music isn't what you'll usually hear in K-Pop.
K-Pop is usually loud and impactful, J-Pop is more like candy music while XG's music is Hip-hop and R&B heavy.
Also I believe if they identified themselves as a K-Pop group but only release English music, it would cause an uproar with Koreans which might be the same with EXP Edition(All Western K-Pop group).
There is no denying though that X-Pop can probably be classified as a sub-genre of K-Pop
